The invention is based on a hydraulic brake system having an anti-skid or anti-lock system (ABS) and traction control (ASR) for motor vehicles.
In a known brake system of this type, which is embodied as a dual-circuit brake system with front/rear axle or front/rear brake circuit distribution (DE 38 16 073 Al; U.S. Pat. No. 4,900,102), the charging valve is embodied as a hydraulically controlled 2/2-way valve and the reversing valve is embodied as a 3/2-way magnet valve. The 2/2-way valve is connected by its control inlet to a brake circuit outlet of the master brake cylinder, and in its uncontrolled basic position it is open. The 3/2-way magnet valve is open in its unexcited basic position and thus allows the brake fluid to flow through the connecting line, and in its working position brought about by magnet excitation, it connects the outlet of the pump element to the brake fluid tank via a pressure limiting valve, by disconnecting the master brake cylinder from the brake circuit. The pressure limiting valve limits the system pressure in the brake circuit during traction control (ASR) to approximately 70 bar. The control valves are embodied as 3/3-way magnet valves, in such a way that in their first, unexcited basic position, a pressure buildup in the wheel brake cylinders can take place; in their second, middle valve position, which is brought about by excitation of the control valves with half the maximum current, the wheel brake cylinders are blocked off so that the pressure in them is maintained; and in their third, terminal valve position, which is brought about by valve excitation with maximum current, the wheel brake cylinders are connected for pressure reduction to the inlet of the associated pump element of the return pump.
The reversing valve embodied as a 3/2-way magnet valve is always switched over whenever at least one of the driven wheels is exhibiting slip. The return pump is switched on at the same time. The return pump element that is operative in the brake circuit of the driven wheels and is embodied as self-aspirating aspirates brake fluid from the brake fluid tank via the opened charging valve and generates a brake supply pressure that is fed via the control valves into the wheel brake cylinders of the driven wheel or wheels that is slipping. The control valve of the nonslipping driven wheel is shifted into its middle position, so that the associated wheel brake cylinder is blocked off, and no brake pressure can be built up. For pressure holding and pressure reduction, the control valve associated with the slipping driven wheel is shifted to its middle or terminal position. Excess brake fluid is returned to the brake fluid tank via the reversing valve and the pressure limiting valve.